The photo-sites in solid state image sensors generate current in the absence of light due the thermal action of electrons in the devices. This thermally generated current is called dark current, and adds to the photo-current generated by the photo-sites when exposed to light. One approach to reducing dark current is to cool the image sensor, for example using liquid nitrogen. This approach is not considered practical for consumer applications. Accordingly, it is generally necessary to correct digital images by removing the component of the image signal due to dark current. To remove the dark current from the image signal, the level of dark current produced by the sensor must be known. If the dark current is under-estimated, the dark current corrected image will appear to have flare and when it is over-estimated shadows will turn black thereby causing shadow detail to be lost. In either case, after white balancing, it is impossible to maintain a neural gray without hue throughout the gray scale when the dark current is improperly determined. White balancing is an adjustment to the color balance of an image to take into account the color of the illuminant under which the image was acquired.
One technique for determining the dark current is to measure a sample of image sensors in the factory to determine the average dark current produced by the sensors, and to employ this value for correction. This will not provide a satisfactory solution in cases where the dark current is temperature dependent or where it changes over time. Another technique for determining the dark current in an image sensor is to mask some of the photo-sites in the image sensor, sense the signal produced by the masked photo-sites and subtract the sensed signal from the signals produced by the unmasked photo-sites in the image sensor. See for example U.S. Pat. No. 5,105,276 issued Apr. 14, 1992 to Schrock.
One problem encountered with this latter approach is that anomalies caused by the device physics may prevent a light shielded pixel from accurately estimating the dark current. Further, other device properties may change the dark current dependent upon the scene brightness or the length of the exposure. For example, in some CMOS image sensors the nonlinear signal response of the photo-sensor at low signal levels makes it difficult to estimate the dark current signal level from the signals produced by shielded pixels.
It is also known to determine correction values for individual images by using the histograms of the images. In these applications, it is assumed a small predetermined percentage of the pixels are black. The next step is to form a histogram of the pixel values and determine the code value that is associated with the predetermined percentage. For example, suppose it is assumed that 2% of all pixels are black and the image being corrected contains 1 Megapixels. This means that 20 k pixels in the image are assumed black. Next, all of the pixels in the histogram are added up starting from code value 0 to n to find the last bin for which the sum is less than 20 k. The correction offset is then set to n. The Kodak Palm Pix camera uses this method to determine a dark level correction for a still image prior to any dark correction.
This approach is not applicable to correct for dark current in a video stream because it would not be stable over time because the value of the offset is determined by an estimate that includes a range of variability. Furthermore, if previously corrected signals are used to determine the offset, the approach would not converge on a reasonable correction because each new application of correction would add to the last, driving the correction to an extreme. This is important since the design of available image sensors often include a dark level correction that is applied on the image sensor chip before the signal becomes available for the further processing that is required for determining the offset using the histogram method.
There is a need therefore for an improved technique for correcting for dark current in a solid state image sensor that avoids the problems noted above.